I am going to add the dissenting opinion to this question, even though it seems current policy SE wide is against what I will say...
If you copy and past verbatim (i.e. no changes or additions) an answer given by another user on another SE site you should make it community wiki.
When should I make my answers Community Wiki?
When you want to enhance the "wiki" aspect of your post, so that it can be a continually evolving source of good information through repeated editing.
When you feel your post would benefit from less concern about voting affecting the reputation of those participating in it.
It is my opinion that when an answer is copied and pasted verbatim that it would fall under both aspects of the usage.
- The CW answer will make the answer freely editable in case the original answer is ever updated. They can then freely update the answer on both sites without having to worry about their edit being rejected or rolledback It also might be especially useful if the user who gave the original answer is active on both site or the user who did copy and pasting become inactive.
- The CW status will also free a voter conscience if they the answer is a good answer but have reservations about the copy/pasted nature of it. This may be a minority of users, but I feel it is at least worth mentioning.
To be perfectly clear this is only when answers are copied/pasted verbatim. If you are "inspired" by another answer, use it as a source, then add your own content or edit the existing content feel free to post as a standard answer. Overall your new answer should have an almost entierly different look and feel to it. It is sometimes easier to see where another answer found a quote or an idea that makes things click in your mind.
Also if you are copy/pasting your own answer, go right ahead!